radiation (radiation measurement) detector generally in the form of a glass plate thinly coated with a transparent medium containing a silver halide compound. Passage of charged subatomic particles is recorded in the emulsion in the same way that ordinary black and white photographic film records a picture. After photographic developing, a permanent record of the paths of the charged particles remains and may be observed through a microscope. Radioactivity was discovered in 1896 by its effect on a photographic plate, and nuclear emulsions later played a pivotal role in cosmic-ray research—for example, in the discovery of the pion in 1947. Emulsions continue to be useful in the study of the production and decay of short-lived particles produced in high-energy particle physics experiments.